1. Field of the Invention
This invention relates to a method for producing a sintered porous bearing and the sintered porous bearing for use in, for example, spindle motors rotating at high speeds and a production method therefor, and more particularly to the method for producing a sintered porous bearing and the sintered porous bearing which eliminates loss of energy due to abrasion by high speed rotation, eliminates increase of abrasion due to vibration and the like and enables to be used in a long-term.
2. Description of the Related Art
Conventionally, a sintered porous bearing (hereinafter referred to as bearing) in which a middle portion along an axis of an internal circumferential portion which contacts a shaft has a larger diameter than that of both ends and is expanded toward the inside of the bearing substance has been known. This type of the bearing is subjected to a small rotation resistance against the shaft because the expanded portion in the middle portion is not in contact with the shaft and therefore suitable for supporting the shaft rotating at high speeds. For example Japanese Patent Publication No. 60-54525 has disclosed a art that by using die having a gap for part of an external circumferential portion of a sintered object, the sintered material having straight cylindrical faces of its internal and external circumferential portions is recompressed so as to expand its middle portion along its axis of the sintered material toward outside in the radius direction thereby forming a middle expanded portion in the bearing substance. The similar arts have been disclosed in Japanese Patent Publication No. 6-4883, Japanese Patent Application Laid-Open No. 62-151502. Further, Japanese Patent Application Laid-Open No. 2-8302, Japanese Patent Application Laid-Open No. 3-240901 and Japanese Patent Application Laid-Open No. 1-242821 have disclosed an art that the internal circumferential portion of a bearing is formed in cylindrical form having steps including a large diameter portion and a small diameter portion and then an end portion of the large diameter portion is drawn so as to reduce the diameter thereof thereby forming a middle expanded portion.
According to the above described conventional arts, the external circumferential portions of the bearings in any cases are not straight because of stepped formation, then mounting those bearings onto a housing is difficult. That is, because the external circumferential portion of the large diameter portion of the bearing is not restrained at the time of recompression, not only a disparity occurs in dimensional accuracy but also the bearing is supported unstably so that a large load cannot be supported when it is mounted on the housing because it is supported only by either the large diameter portion or the small diameter portion. Further, with respect to art of reducing the diameter by drawing the end of the large-diameter portion, a difference occurs in internal porosity because a degree of processing differs between both ends of the bearings.
Additionally, Japanese Patent Publication No. 63-43611 and Japanese Patent Publication No. 4-45962 have disclosed such art that first a concave portion is formed such that it extends around an entire circumferential portion of a middle portion in axial direction of an external circumferential portion of a bearing by mechanical processing and then by recompressing the bearing in the axial direction, the concave portion is made to expand toward the outside so as to form an expanded portion on an internal circumferential portion thereof. However, because the sintered porous bearing is superior in some respect to other bearings because it can be produced at a relatively low cost, cutting processing is actually impossible in viewpoints of production cost.
Further, Japanese Patent Application Laid-Open No. 2-107705 has disclosed an art that at the time of sizing, a large diameter portion formed on a core is positioned in a middle portion in the axial direction of a bearing, the bearing is compressed along the axial direction, the bearing is drawn out of a die together with the core and the core is pulled out of the bearing. According to this art, although an external circumferential portion of the bearing can be formed in straight form, because the bearing is subjected to elastic deformation when the core is pulled out, a dimension of the large diameter portion formed on an internal circumferential portion of the bearing can not be set large and further setting of the density of the bearing is also restrained.
According to Japanese Utility Model Application Laid-Open No. 61-124702, a concave portion is formed around an entire surface of a middle portion in the axial direction of an external circumferential portion of a bearing, this bearing is pressed into an inside of a housing case and the diameter thereof is reduced corresponding to a distance of the pressing so as to form a middle expansion in an internal circumferential portion thereof. However, this art has such disadvantages that a high precision is required in the internal dimension of the housing case and material thereof is limited.
According to the above arts, contact area is reduced by forming the middle expanded portion on the internal circumferential portion of the bearing. To attain the same effect, for example Japanese Patent Publication No. 42-15447 has disclosed an art that a plurality of longitudinal grooves extending in the axial direction are formed on an internal circumferential portion thereof. According to Japanese Patent Publication No. 44-15683, a plurality of longitudinal grooves are formed on an internal circumferential portion of the bearing and after that, by correcting the entire internal circumferential portion to a real circular shape as viewed in section, a density of a portion having grooves is adjusted so as to be low and a density of a portion having no grooves is adjusted so as to be high. Further, Japanese Utility Model Publication No. 47-36739 has disclosed a bearing in which the density of a portion containing a longitudinal groove is made low and at the same time the density of a portion between the longitudinal grooves is made high. Further, Japanese Patent Publication No. 47-50899 has disclosed a bearing in which a sectional shape of an internal circumferential portion thereof is constituted of plural circles having a larger curvature than a radius of a shaft. In this bearing, clearance between the bearing and the shaft varies along its circumferential portion and the shaft is supported by hydraulic pressure produced when lubricant is sucked from a portion having a large clearance to a portion having a small clearance.
A number of arts similar to the aforementioned bearings have been proposed and further various groove types such as angled grooves and spiraled grooves have been proposed. A common purpose of these proposals is to reduce a rotational in resistance of a shaft or concretely speaking, a resistance (resistance by fluid lubrication) between an internal circumferential portion of a bearing and an external circumferential portion of a shaft which is a particular problem in high speed rotations. However, although such an object is attained by these proposals, they have such disadvantages that hydraulic pressure drops because lubricant leaks outside from both ends of the bearing through its grooves and lubricant is seriously consumed. These result in drop of bearing performance. Although various kinds of bearings for high speed rotation have been proposed since before, any one of them has its own disadvantages.